In the art of making micro-crystal cadmium sulfide voltaic cells it has been the practice to fabricate the cadmium sulfide layer of considerable thickness, say 20 microns. This has been deemed necessary to assure that pin holes, or other types of defects, do not occur in the layer which, if present, render the cell inoperative. It has heretofore been considered unfeasible to utilize extremely thin layers of cadmium sulfide because a large proportion of the cells prove defective in practice. It is one purpose of the present invention to provide a large area photo-voltaic cell capable of being incorporated in a system employing areas of photo-voltaic generators covering areas of the order of square miles, to enable large scale production of electric power. In such systems the total quantity of cadmium required becomes a problem, since cadmium is in short supply in the United States and is expensive. Reduction of the feasible thickness of cadmium compound required to fabricate a given area of cell is therefore crucial economically, and a reduction of thickness of CdS layer by an order of magnitude or more renders economically feasible a large scale power generator of the photo-voltaic type, which otherwise is not economically feasible. Utilization of minimum cadmium per unit area of cell is rendered feasible by utilization of a rectifying like positive electrode in the cell and by irradiating the cadmium sulfide layer during the formation process with intense ultra-violet light.
It is, accordingly, a primary object of the present invention to provide a photovoltaic cell which utilizes minimum weight of cadmium per unit area and which can therefore be economically utilized as a power source in a large scale electrical power generation system. This same objective is subserved by providing a cell which has only coplanar electrodes, and also in terms of time required to fabricate a given area of cell, a twenty micron layer requiring twenty times as much spray time as does a one micron layer, in forming the requisite cadmium sulfide microcrystalline layer on a substrate.
In the U.S. Pat. No. to Hill et al 3,148,084, issued Sept. 8, 1964, a method is taught for forming a layer of cadmium sulfide microcrystals on a glass substrate. Essentially, the method involves spraying the glass substrate, while the layer is hot, with a cadmium salt-thiourea complex, i.e., cadmium chloride plus a thiourea, in suitable proportions. The teaching of the patent is that the glass may be heated by means of a hot plate, and that the spraying may take place in the atmosphere. Whe have found that precisely uniform temperature of the glass plate is essential and that a hot plate is not able to heat a glass plate uniformly because the hot plate and the glass plate do not make perfect contact throughout, and that even slight non-uniformities of temperature of the glass substrate produce anomalous areas of the layer of CdS, which can render an entire photovoltaic cell inoperative. The layer of CdS must grow in the form of many tiny crystals, the axes of which are predominantly parallel. Application of the sprayed materials at a uniform, and sufficiently slow rate, is important, as is uniformity of temperature, to assure uniformity of crystal growth rate and of orientation over the entire glass plate. We have found that application of very intense ultraviolet light over the entire CdS microcrystalline layer, as it grows, to enhance uniformity and orientation of crystal growth improves the end produce, as evidenced by the fact that the percentage of plates which prove imperfect is reduced. The layer of CdS may be only about 1.0 to 2.0 microns thick, in the process of the present invention, which is contrary to prior art practice.